The importance of chain terminating or end-capping copolyester-carbonates by means of certain molecular weight regulating or chain terminating compounds is well known. Copolyester-carbonates which are not end-capped or chain terminated are generally insufficiently heat stable since the free phenolic end groups provide a reactive site which is detrimental to the stability of the copolyester-carbonate polymer. Furthermore, the absence of a chain terminating or end capping agent or condition during the preparation of the aromatic copolyester-carbonate results in a polymer with a molecular weight which is often so high that the polymer is too viscous to mold at reasonable temperatures.
Standard chain terminating agents employed in the preparation of aromatic copolyester-carbonates include members of the phenol family such as phenol itself and tertiary butyl phenol. While these phenol chain terminating agents are generally effective, there is a need, in certain applications, for copolyester-carbonates exhibiting higher heat resistance, particularly higher heat distortion temperatures, than those exhibited by copolyester-carbonates employing phenols as end capping agents. It is known that replacing these phenol chain terminating agents by other compounds, such as benzoate esters, results in copolyester-carbonates exhibiting improved heat resistance.
However, the area of chain terminators and their effects upon the properties of copolyester-carbonates is generally not completely understood and is one wherein the empirical approach is generally the rule rather than the exception in determining whether a particular compound or class of compounds will function effectively as chain terminators for copolyester-carbonates. Thus, for example, while one particular compound may function effectively as a chain terminator for copolyester-carbonates another similar compound or class of compounds might be ineffective as a chain terminator. This art is further complicated by the fact that not only must a particular compound function as a chain terminating agent, but this compound, when incorporated into the copolyester-carbonate polymer chain as a terminal group, must not adversely affect the positive and advantageous properties of the copolyester-carbonate. Thus, while some compounds may be effective chain terminators they may not be practical since they adversely affect some of the positive and advantageous properties of copolyester-carbonates.
It is, therefore, an object of the instant invention to provide novel chain terminated copolyester-carbonates exhibiting improved heat resistance while simultaneously generally retaining, to a substantial degree, substantially all or most of their other advantageous properties.